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Hurricanes: New Orleans Under Threat

Long before Hurricane Katrina, most experts knew the city of New Orleans was vulnerable. Situated up to 3.7 meters (12 feet) below sea level, between a lake and a river, and on the hurricane-prone Gulf Coast, the city was arguably a disaster waiting to happen. This video segment adapted from NOVA scienceNOW describes the threat Hurricane Ivan posed just one year before Katrina — an ominous foreshadowing of one of the country's worst natural disasters.

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Humans have been altering the natural environment to suit their needs for thousands of years. We've constructed shelter where none existed, planted food crops where forests once flourished, and in one of the more striking examples, built an entire city in the middle of a swamp. New Orleans, Louisiana, is a testament to the human will and ingenuity required to overcome seemingly impossible odds — in a part of the world where a great deal of commerce stood to be gained. However, in August 2005, Hurricane Katrina tragically illustrated the potential consequences of defying the forces of nature.

New Orleans sits in the middle of an ancient floodplain of the Mississippi River. Periodically, this mighty river, like most wild rivers, overflowed its banks, inundating the surrounding area. What was a natural and necessary occurrence for the wetland ecosystem of the Gulf coast threatened the viability of communities along the banks of the Mississippi. In response to this threat, people, as part of a government plan, began building a system of levees to heighten the river's banks and protect themselves from flooding.

Ironically, the centuries-old effort to control seasonal river flooding in New Orleans and other coastal cities has made the same communities increasingly vulnerable to ocean flooding from tropical storms and hurricanes. Wetlands along the Gulf coast provide the first line of defense against deadly hurricanes, by both limiting the storms' access to the warm open ocean water that drives them and by creating a physical barrier to the floodwaters, or storm surges, that they generate. Yet, coastal wetlands have been deteriorating and shrinking for decades because they no longer receive the influx of sediment and nutrients that periodic river flooding once provided.

The bottom, or substrate, of coastal wetlands is unstable. It settles and sinks continuously. Without a steady supply of new sediment and nutrients, wetland plants soon become flooded themselves, causing expanses of open water to form. As a result of Mississippi River flood control, together with the opening of shipping channels that have caused saltwater to infiltrate freshwater wetlands, Louisiana has lost nearly 5180 square kilometers (2,000 square miles) of coastal wetlands, an area the size of the state of Delaware, in the last 70 years. As this trend continues, the likelihood that another hurricane will devastate the Gulf Coast region only increases.